Hi there.
I haven't done this on redwood but I have airlayered from a few of my trees just for experimentation purposes.
From my experience, as long as its done at the start of spring, using the correct rooting compound (hard or semi hard wood), and the moss is always kept moist, you should start seeing fine roots through the wrapping in about 2-3 months.
If the branch looks like its drying out or leaves have changed colour/fallen off then the branch is dying and there is little you can do
You should carefully check the cabium and the cuts that you've wrapped for any decay or to see if the cabium managed to grow over the cuts.
If there are fine roots developing and you move the moss or wrapping you could break the new fragile roots so be extra careful.
Good luck and I hope this helps.
A.J